This invention relates to a fiber optic polarization controller which is capable of continuously controlling the polarization state of polarized light propagating through an optical fiber to a desired state.
A polarization controller is one of the indispensable optical devices in such fields as optical communication, optic sensors, optical fiber gyroscopes, optical signal processing, etc. which use single-mode fibers and where polarization of the light propagating therethrough causes a problem. Among various devices, fiber optic polarization controllers are advantageous as they are low in loss, simple in structure and easy in handling.
FIG. 1 shows a structure of a prior art optic fiber polarization controller. The controller was disclosed by H. C. LeFevre in Electronics Letters (9/25/1980) Vol. 16, No. 20 wherein a single-mode fiber 1 is wound on the side of a bobbin 10 to cause a change in refractive index inside the fiber by utilizing an inner stress generated by curvature. As the birefringence principal axis rotates simultaneously with the bobbin 10 as it rotates in the direction R, incident polarized light P.sub.in can be controlled to have a desired polarization in outgoing light P.sub.out. The positions 2A, 2B of input and output sections of an optical fiber 1 are fixed with external fixing means. As the fiber 1 is also fixed on the bobbin 10, the rotation R of the bobbin 10 causes twist at A, B, near the positions 2A, 2B. In order to prevent twist breaking of the optic fiber 1, it is necessary to set a limit on rotation angle of the bobbin 10. It was therefore impossible to realize polarization control function which could continuously follow all the changes in polarization in an optic fiber.